The present invention relates to an improvement in a field emission type or surface ionization type ion source suitable for use in an ion microbeam milling system and an ion microimplanter for fabricating ultra large scale integrated circuits, and in a microbeam analysis system.
FIGS. 1 and 2 show two examples of conventional ion sources. The ion source shown in FIG. 1 is mainly made up of an emitter chip 1 attached to the turning point of a hairpin filament 2 and having a sharp tip, an extraction electrode 4, and an ion accelerating/extracting power supply 6. The operation principle of this ion source is as follows. First, an ion source material 3 is supplied to the junction part of the hairpin filament 2 and the emitter chip 1. A current is forced to flow through the filament 2, to melt the ion source material 3 by Joule's heat. Thus, the ion source material 3 is supplied to the tip of the emitter chip 1 in a molten state. Next, a voltage is applied between the extraction electrode 4 and emitter chip 1 by means of the power supply 6, to draw out an ion beam 5 from the tip of the emitter chip 1 on the basis of the field emission or surface ionization.
The above ion source has the following drawbacks.
(1) The ion source material is heated by Joule heating, and therefore it is not possible to heat the ion source material to temperatures exceeding a limit.
(2) The molten ion source material 3 is supported by the emitter chip 1 and filament 2 in an open state, and therefore evaporates violently. Accordingly, surrounding members are greatly contaminated with the material 3, and moreover the useful life of the ion source is short.
(3) In the case where a chemical compound is used as the ion source material 3, the composition of the vapor of the ion source material 3 varies with time on the basis of a difference in vapor pressure between constituent elements, and therefore the intensity of an ion current varies with time.
(4) The amount of the ion source material 3 loaded in the ion source is determined only by the surface tension of the molten ion source material. Accordingly, a limited quantity of ion source material can be loaded in the ion source, and therefore the useful life of the ion source is short.
FIG. 2 shows a field emission type ion source recently developed by part of the present inventors (Japanese Utility Model Kokai (Laid-Open) No. Sho. 56-123453), which makes use of electron bombardment heating. The ion source shown in FIG. 2 includes an emitter chip 1, a lid 8 for preventing the evaporation loss of an ion source material 3, a crucible 7, a filament 2 for electron bombardment heating, a control electrode 9, an ion extracting electrode 4, a filament heating power supply 11, an electron accelerating power supply 10, and an ion accelerating power supply 6. The operation principle of this ion source is as follows. First, the ion source material 3 is loaded in the crucible 7. Then, the emitter chip 1 is heated by the electron beam from the filament 2, to melt the ion source material 3 on the basis of the thermal conduction from the emitter chip 1. Finally, an ion extracting voltage is applied between the emitter chip 1 and ion extracting electrode 4, to extract an ion beam 5.
The ion source shown in FIG. 2 is superior to the ion source shown in FIG. 1 and heated by a Joule heat, in that the ion of a material having a high melting point and the ion of a reactive material can be obtained, but still has the following drawbacks.
(1) In the case where a material having a high vapor pressure is used as the ion source material 3, the ion source material 3 evaporates too violently to make it difficult to supply the ion source material to the tip of the emitter chip 1.
(2) Since the ion source material 3 is heated from the outside with the irradiation of electron beam, it is difficult to heat the ion source material 3 uniformly. Accordingly, it is difficult to supply the ion source material 3 to the tip of the emitter chip 1.
(3) The ion source material 3 having a high vapor pressure readily evaporates, and flies off in various directions. Accordingly, it is very dangerous to use a noxious material as the ion source material.
(4) It is difficult to use a less reactive emitter chip which is made of an insulating material.